Recently, thick film circuit board devices have been used in electronic appliances for constructing hybrid integrated circuits. In the hybrid integrated circuit, the thick film circuit board device has film resistors and film conductive wirings, both formed on the surface of an insulative base board by printing, e.g., screen printing. The film resistor is formed of a resistive material printed on the base board. The film conductive wiring is formed of a conductive material printed on the base board. Prescribed circuit elements such as capacitors, transistors or the like are mounted on the base board and are coupled to the film conductive wirings. The film conductive wirings electrically connect the film resistors and/or the circuit elements with each other. Thus, a hybrid integrated circuit results.
The thick film circuit board device has been widely accepted in order to reduce the weight and size of electronic appliances.
Referring now to FIG. 1, a conventional method for manufacturing such a thick film circuit board device will be described. The conventional method comprises the steps of preparing an insulative base board 10 made of ceramic, such as alumina (Al.sub.2 O.sub.3), and providing a resistive layer 12 and a conductive layer 14 on base board 10. Resistive layer 12 and conductive layer 14 are formed by screen printing of a resistive paste and a conductive paste on base board 10, respectively. The resistive paste may contain, for example, ruthenium oxide (RuO.sub.2) powder and glass frit. The conductive paste may contain, for example, copper (Cu) powder or silver-palladium (Ag/Pd) powder.
First, the resistive paste is screen printed on base board 10, dried and set at a temperature of about 800-900.degree. C. in an atmosphere of air or oxide gas, so that resistive material layer 12, i.e., the film resistor is formed. Then the conductive paste is screen printed on base board 10, dried and set at a temperature of about 500-700.degree. C., so that a set of conductive layers 14, i.e., the conductive wires are formed. The setting of the conductive paste is carried out in an atmosphere of inactive or resolutive gas, such as nitrogen gas.
A prescribed pair of conductive layers 14 to be coupled with resistive layer 12 are overlapped with their ends overlapping on the ends of resistive layer 12. The resistance of resistive layer 12 is then adjusted by trimming the layer with YAG laser beam or by sandblasting.
An insulative film made of silicon resin is screen printed o resistive layer 12 and/or conductive layer 14, and then hardened at a temperature of about 100-120.degree. C., so that a solder resist film 16 is formed. Solder resist film 16 covers the thick film circuit board device, except portions to be connected to a circuit element 18 . Circuit element 18 is coupled to prescribed conductive layers 14 by soldering with solder 20.
The thick film circuit board device can be provided with lead pins (not shown) for connection with foreign circuits. Also, the thick film circuit board device and circuit element 18 mounted thereon can be covered with another insulating film (not shown) made of resin for protecting &he entire surface of the circuit board device and circuit element 18.
However, the above-mentioned conventional method for manufacturing such a thick film circuit board device has a drawback, as described below. Conductive layer 14 is formed on base board 10 by screen printing after the formation of resistive layer 12 The screen printings of resistive layer 12 and conductive layer 14 are carried out using screen printing plates. On screen printing plate for printing conductive layer 14, the gap between the prescribed pair of conductive layers 14 to be coupled to resistive layer 12 is shorter than the length of resistive layer 12. Thus, respective ends of prescribed conductive layers 14 are coupled to the ends of resistive layer 12.
When a shear or shift occurs between the base board and screen printing plate during either of the printings for resistive layer 12 and conductive layer 14, the overlap joining conductive layer 14 to resistive layer 12 may occur, or the amount or range of the overlap may become insufficient. In the case of a shear which results in an insufficient overlap, the problem is difficult to discover. This is because the portion of resistive layer 12 corresponding to the gap between conductive layers 14 may be displaced in response to the shear but the length of the portion does not vary.
Therefore, a thick film circuit board device manufactured by the conventional method is subject to inferior coupling between resistive layer 12 and conductive layer 14.